1. Field of the Invention
The present invention relates to an image forming apparatus which is capable of performing double-sided printing and outputting a job mixedly involving single-sided printing and double-sided printing at a high speed.
2. Description of the Related Art
Conventionally, to perform efficient double-sided printing on sheets, U.S. Pat. No. 4,978,980 has proposed the following method: First-side image formation is continuously performed on a predetermined number of sheets, and then, after the sheets each having an image formed on a first side thereof are circulated through a double-sided conveying path, second-side image formation on the sheets and first-side image formation on newly fed sheets are alternately performed (which is called “alternate sheet feed”).
Further, in the field of commercial printing, a print job mixedly involving single-sided printing and double-sided printing is generally handled as one set, and high-speed output is demanded of such a job. In the control method disclosed in U.S. Pat. No. 4,978,980, it takes time from completion of image formation on the front side of a first sheet for double-sided printing to start of image formation on the reverse side of the sheet having been circulated through the double-sided conveying path. Whenever single-sided printing is switched to double-sided printing, the certain time is taken, and hence total printing speed is considerably reduced.
To solve this problem, Japanese Patent Laid-Open Publication No. 2004-145218 has proposed the following method: the first-side page of a double-sided printing sheet following a single-sided printing page group is subjected to image formation prior to the single-sided printing page group, the double-sided printing sheet is moved into a double-sided conveying path and kept on standby, and then the single-sided printing page group is printed and discharged. Thereafter, the second-side page of the double-sided printing sheet, remaining to be printed, is printed and then the sheet is discharged. Thus, a job mixedly involving single-sided printing and double-sided printing is output at a high speed.
As described above, only the first side of the double-sided printing sheet, which is to be output as a page following the single-sided printing page group, is printed prior to the single-sided printing page group, and then the single-sided printing is executed. This makes it possible to execute single-sided printing in parallel with double-sided printing, while making effective use of a time period over which a sheet having undergone first-side image formation for double-sided printing is conveyed through the double-sided conveying path, whereby total printing time can be reduced dramatically in comparison with the conventional methods.
A description will be given, with reference to FIG. 3, of an exemplary case where a job is executed for printing one double-sided printing sheet (sheet S1), one single-sided printing sheet (sheet S2), one double-sided printing sheet (sheet S3), one single-sided printing sheet (sheet S4), and one double-sided printing sheet (sheet S5) in the mentioned order. In FIG. 3, the front and reverse pages of the sheet S1 are denoted by P11 and P12, respectively, the page of the sheet S2 by P2, the front and reverse pages of the sheet S3 by P31 and P32, respectively, the page of the sheet S4 by P4, and the front and reverse pages of the sheet S5 by P51 and P52.
As shown in the upper part of FIG. 3, in a case where the pages are subjected to image formation in the page order, sheet conveyance time is needed between front-side image transfer and reverse-side image transfer, i.e. between image transfer on the page P11 and image transfer on the page P12, between image transfer on the page P31 and image transfer on the page P32, and between image transfer on the page P51 and image transfer on the page P52, and therefore a time interval between image forming operations increases. To solve this problem, the pages P31 and P51 for double-sided printing are subjected to image formation prior to the page P2 for single-sided printing, as shown in the lower part of FIG. 3, whereby it is possible to perform single-sided printing while making effective use of the sheet conveyance time between front-side image transfer and reverse-side image transfer, to thereby reduce the total printing time (this control will be hereinafter referred to as “the passing control”).
Now, let it be assumed that in the above-mentioned passing control, a state (e.g. a sheet-absent state) has occurred in which sheets for use in single-sided printing cannot be fed after an image has been formed on each of the front sides of respective sheets for double-sided printing, prior to single-sided printing, as shown in FIG. 4. In this case, when reverse-side image formation for double-sided printing prior to single-sided printing is continued, a product without passed pages for single-sided printing is created. In short, the printed pages come to be out of order. Therefore, it is required to stop the printing operation, but if the printing operation is stopped, the sheets S3 and S5 are left in the apparatus, and hence the operator has to remove the sheets, which degrades operability.
If the sheets S3 and S5 are left on the conveying path until sheets for the passed or overtaken single-sided printing page can be fed, the sheets S3 and S5 can curl on the bent conveying path and become unusable as products. Therefore, it is required to remove the sheets S3 and S5.
Further, there is a case where due to a change of image forming conditions between pages, the pages cannot be continuously printed or printing thereof is temporarily suspended. For example, a target temperature of a fixing device can be changed when single-sided printing on a plain sheet is switched to double-sided printing on a thick sheet. Further, an adjustment operation for maintenance of image quality can be performed when single-sided printing in a monochrome printing mode is switched to double-sided printing in a full-color printing mode.
In such a case, if the first sides of respective sheets for double-sided printing are printed prior to a single-sided printing page group, printing conditions are changed between printing on the first side of each double-sided printing sheet and printing on the second side of the same, which can cause differences in color hue and glossiness between the front and reverse sides of the sheet.